Radio system



Decu 3, 1940.. A. v. BEDFoRD 2,223,982

RADIO SYSTEM Dec. 3, 1940. A. v. BEDFORD RADIO SYSTEM Filed Jan. 29, 1937 2 Sheets-Sheet 2 Snnentor aya/'a5 (Ittorneg Patented Dec. 3, 1940 UNITED STATES PATENT OFFICE RADIO SYSTEM Delaware Application January 29, 1937, Serial No. 122,898

7 Claims.

My invention relates to short wave radio systems and particularly to radio systems for the transmission and reception .of pictures.

An object of my invention is to provide an improved method of and means for reducing or substantially eliminating at a radio receiver the effects of an undesired modulation, such as cross-modulation, of the transmitted carrier Wave.

Such an undesired modulation may be produced by the metal propeller of an airplane where a television signal is being transmitted from the airplane.

A further object of my invention is to provide an improved television receiver.

My invention will be vdescribed specically in connection with a television system in which the transmitter is located on an airplane. In such a television system, it has been found that when the metal propellers von the airplane are about one-quarter wave length long from the hub to the tip of the blade for a certain wave length used in transmission (50 megacycles, for example) the received field strength on the ground is greatly altered by the change in the angular position of the propellers. In the particular installation being described, the propellers were of the two-blade type and, during flight, rotated about 1500 R. P. M. thereby modulating the television carrier and its television modulation component at about 50 cycles per second. The magnitude of such propeller modulation is a variable amount up to about 50 percent modulation, depending upon the positionof the antenna and the direction of the receiving point from the airplane.

It has been found that when the transmitted signal includes a pilot signal, or, in the specific system to be described, control or synchronizing impulses, which normally are of constant amplitude, it is possible to reduce .or eliminate the effect of such cross-modulation by introducing at the receiver a modulation derived from said pilot signal which is equal in amplitude, but opposite in polarity, to the undesired modulation.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a View showing a television system embodying my invention,

Figures 2 and 3 are diagrams which are referred to in explaining the invention, and

Figure 4 is a circuit diagram of a television receiver constructed in accordance with another embodiment of my invention.

(Cl. T28-'7.5)

Referring to Fig. l, my invention is shown applied to a television system in which the transmitter, indicated at I, is of the type described in British Patent No. 407,409. In such a transmitter a horizontal synchronizing impulse is 5 transmitted at the end of each scanning line and a vertical synchronizing impulse is transmitted at the end of each picture frame. All of these synchronizing impulses are of the same amplitude, the vertical synchronizing impulses being of greater duration than the horizontal synchronizing impulses, in order to facilitate frequency separation of the two groups of synchronizing impulses at the receiver. In this transmitter the synchronizing signals are of the' same 15 polarity as picture signals in the black direction and have a greater amplitude than the maX- imum amplitude of pictures in the black direction. The composite signal comprising picture signal and synchronizing impulses is caused to modulate the carrier Wave in such direction that the synchronizing impulses cause the carrier Wave to have maximum amplitude.

The signal which is radiated from the transmitter before the undesired propeller modulation or the like has occurred is shown in Fig. 2. The picture signal is indicated at 2 and the horizontal synchronizing impulses are indicated at 3, the vertical synchronizing impulses not being shown. The horizontal synchronizing impulses occur at the rate of 10,200 per second and the vertical synchronizing impulses occur at the rate of per second in the specic system being described, all of the synchronizing impulses being of constant fixed amplitude in the absence of undesired modulation.

In Fig. 3 there is shown the transmitted signal after it has been modulated by the airplane propellers and as it appears at the receiver antenna. It will be noted that such modulation causes the synchronizing impulses to be of different heights, as well as causing the envelope of the wave to have a sine Wave contour. That is, the propeller modulation modulates the synchronizing modulation to produce what is known as cross-modu- 45 lation. It will be apparent that the drawings have been simplified by showing only a few of the synchronizing impulses that occur during one cycle of propeller modulation.

The receiver shown in Fig. 1 is of the same 50 general type described in the above-mentioned British patent. The cathode ray tube, the deilecting circuits, and the Video frequency amplifier are indicated generally at 6. The receiver portion which amplies and demodulates the received carrier wave for supplying the video signal to the unit 6 preferably is of the superheterodyne type comprising a first detector 1, a tunable oscill-ator 8, a plurality of intermediate frequency ampliers 9, I I and I2, tuned to about II megacycles, and a second detector I3. The I. F. stages II and I2 and the I. F. stage I2 and detector I3 are shown coupled by the usual tuned I. F. transformers I4 and I6, respectively. The detector I3 may be 4of the double-wave rectifier type illustrated, the video signal appearing acrossan output resistor I1.

The particular receiver shown in Fig. 1 is provided with the usual automatic volume control circuit, as well as with a special control circuit for introducing the reverse polarity modulation which cancels out the propeller modulation. The A. V. C. circuit, which is conventional in design, includes a direct current amplifier I8 having its input circuit connected across the output resistor I1. 'I'he output of the amplifier i8 is supplied to the I. F. amplifiers 9 and II through resistors I9 and 2| to control the amplification of said ampliers, condensers 22 and 23 being provided to give the desired time constant for proper A. V. C. action. This time constant is such that the A. V. C. circuit will not respond to quick changes in signal amplitude, such asv those occurring in le second or less.

'Referring now to the aboveementioned special control circuit, in accordance with one embodiment of my invention, I supply either a radio frequency or an I. F. signal from the output of amplifier such as amplifier I2 to an amplifier 26, and through an I. F. transformer 21 to a vacuum tube 28. The connection between the transformer 21 and the control gridl 29 of tube 28 includes a grid condenser 3|. A grid resistor 32 which has a high resistance, 4 megohms, for example, is connected between the grid 29 and the cathode 33 of tube 28. As will be explained later, the time constant of the circuit including condenser 3| and resistor 32 is such as to put a bias on grid 29 which varies with the envelope of the received synchronizing impulses. The voltage supplied to the plate of tube 28 is such that the change in plate current is linear with respect to this varying grid bias.

The amplifier 28 is coupled to an amplifier 34 through which the comparatively low frequency signal corresponding to the undesired modulation is supplied to an I. F. amplifier tube for varying its grid bias. In the output circuit of tube 28 a by-pass condenser 36 is provided for filtering out the I. F. carrier.

The propeller modulation signal is supplied, in the specific circuit illustrated, from amplifier 34 through a conductor 31, a coupling condenser 38 and a filter resistor 39 to the control grid of amplifier tube I2. A suitable biasing voltage is supplied to the control grid of tube I2 through a suitable grid leak resistor 90 and through the resistor 39. A filter condenser III is provided which, together with resistor 39, functions to filter out any I. F. carrier which may not be removed by the by-pass condenser 36. Condenser 4| also'` keeps the low end of the secondary of I. F. transformer I4 to ground so far as I. F. in the transformer I 4 is concerned.

Since the amplifier tube I2 is of the usual eX- ponential or variable-mu type employed where the gain is to be controlled, and since the tube is operated on the non-linear part of its grid voltage-plate current characteristic, the propeller modulation signal supplied over .conductor 31 modulates the I. F. signal being supplied to amplier I2. The number of amplifier stages in the special control circuit 23-33 is so chosen that this modulation is of opposite polarity to the propeller modulation already on the I. F. carrier. When grid 29 of tube 28 goes more negative, due to drawing more grid current, the bias on tube I2 goes more negative, thus reducing the gain of tube I2. By making the amplitude of the bucking modulation the same as that of the modulation to be removed, the television signals will be substantially free of propeller modulation at the unit 6.

As previously stated, the time constant of the circuit comprising grid condenser 3| and grid resistor 32 is such as to cause the undesired modulation signal to appear in the plate circuit of tube 28. The action of this portion of the circuit is as follows:

The I. F. signal supplied to the grid 29 has such polarity that the synchronizing impulses, that is, the I. F. carrier representing these impulses, drive the grid 29 positiv-e whereby condenser 3| is charged due to electrons from the cathode of tube 28 reaching grid 29. This charge leaks olf condenser 3| through resistor 32, but the time constant is-such that the charge does not leak ofi much between successive horizontal synchronizing impulses. On the other hand, the adjustment is such that the charge leaks off condenser 3| rapidly enough to permit the Voltage on grid 29 to follow the envelope of the synchronizlng impulses. It should be noted that the tube 2B does not demodulate the I. F. carrier to produce video signals. This is because the time constant of the circuit 3|-32 and the other circuit constants for tube 28 are such that the picture modulation 2 (Fig. 3) varies the grid 29 over a linear portion of the tube characteristic since the amplitude of the I. F. carrier is held within the operating limits of the circuit.

In the system just described the time constant of the circuit including condenser 3| and resistor 32 is about .0012 second. Since this time is about eight times the interval between adjacent synchronizing impulses, there will be no appreciable change in bias between synchronizing impulses. However, it should be noted that this time constant permits the bias to change in accordance with the low frequency propeller modulation.

In this system the grid-cathode conductivity of tube 28 is such when the grid 29 is positive that even a small fraction (for example, /oo) of this conductivity is high compared with the conductivity of the grid leak 32. This is desirable because grid 29 is not positive for the entire duration of a synchronizing impulse but, instead, is positive only for the short duration of the peaks of the I. F. sine wave carrier. The horizontal impulses themselves have a duration of about 1A@ of the interval between them.

In Fig. 4 there is shown an embodiment of my invention in which the bucking modulation signal is derived by means of a diode rectifier and in which this signal serves both for bucking out undesired modulation and for supplying the desired automatic volume control. In Figs. 1 and 4, like parts are indicated by the same reference numerals.

Referring to Fig. 4 it will be seen that the I. F. signal is supplied through the amplifier 26 and transformer 21 to a diode rectifier 46 having an anode 41 and a cathode 48. The lower end of the secondary of transformer 21 is connected to the cathode 48 through a resistorl 49-and voltage supply such as battery 5|, the battery being provided to prevent signals below a given amplitude from causing a iiow of current through the diode. f

A by-pass condenser 52 lis provided, the circuit including this condenser and the resistor`49` having substantially the same time constant as the grid condenser-grid'resistor circuit 3|-32 in Fig. 1. Also, the diode 46 is a low resistance* device, its resistance being about 400 ohms when the electrode 41 is positive. The resistor 49, on the other hand, has a resistance of vab`out4 megohms. It follows that even though thel diode 46` is conducting only a small percentage ofthe time, it Will supply the necessary charging current to condenser 52. Therefore, there is a current flow through resistor 49 which follows the envelope of the synchronizing impulses. There are no pic# ture signals in the output of diode-46 sincethe negative voltage applied to anode 41 by the charge on condenser 52 prevents the picturefmodulation from causing a flow of 4diode current.Y

The voltage drop across resistor 49, or'across a portion thereof, is supplied through a filter resistor 53 and a conductor 54 to the control grids of the amplifier tubes in one or more I. F. amplifier stages such as amplifiers 9 and I i. This voltage is of the proper amplitude and polarity to substantially buck out the propeller modulation as described in connection with Fig. 1.

Preferably an additional control voltage of less amplitude is supplied to the control grid of I. F. amplifier I2 to buck out the propeller modulation more exactly. This voltage is taken from a point on resistor 49 and supplied through a resistor 56, switch 51 and conductor 58 to the said control grid. By moving switch 51 to the contact point 59, the amplifier tube I2 has normal bias applied thereto and a bucking voltage is supplied to amplifiers 9 and Il only. The condensers 6l and 62 cooperate with resistorsv 53 and 56, respectively to function as filters for removing I. F. signal.

Attention is called to the fact that in Fig. 4 the I. F. signal for amplifier 26 is supplied from the output circuit of amplifier Il rather than from the output circuit of amplifier I2'. Therefore, the signal supplied through conductor 58 controls the gain of an amplifier following the point from which the control signal is derived. This makes itpossible to buck out the propeller modulation completely by a proper adjustment of the circuit.

The values of various circuit elements have been indicated on the drawings in ohms,megohrns, microfarads, and micro-microfarads. It is to be understood that these values are merely given by way of example and that they may be varied Within wide limits.

While I have shown the control voltage or voltages applied to I. F. amplifiers, it should be understood that they may be applied to radio frequency amplifiers or to a balanced amplifier or video signals, such an amplifier comprising two exponential tubes connected in push-pull.

I claim as my invention:

1. A television system comprising means for transmitting a carrier Wave modulated by picture signals by synchronizing impulses occurring at a comparatively high frequency and by framing impulses occurring at a comparatively low frequency, said impulses having a greater amplitude than said picture signals and said modulation being in such direction that said impulses increasefthe amplitude of said carrier wave, a television receiver having means for intercepting said modulated'carrier Wave after an undesired low-frequency modulation has been superimposed thereon, the frequency of said undesired modulation being of the order of the frequency at 4wlfiichsaid framing impulses occur, means for demodulating said carrier Wave to produce picture signals, an amplifier preceding said demodulating means and comprising an amplifier tube having 'a plurality of electrodes including a control grid and having a non-linear grid-voltage plate-current characteristic, means for deriving from said intercepted signal at a point preceding said demodulating means a low frequency signal corresponding to said undesired modulation, and means for supplying said low frequency signal to said control grid with such polarity and amplitude as to substantially reduce said undesired modulation.

`2.` A television system comprising means for transmitting a carrier Wave modulated by video signals including picture signals and periodically recurring high frequency synchronizing impulses and 10W frequency framing impulses, said impulseshaving` a greater amplitude than said picture signals and said modulation being in such direction' that said impulses increase the amplitude ofsaid carrier wave, a television receiver having means for intercepting said modulate carrier wave after an undesired low frequency modulation has been superimposed thereon, said low frequency being of the saine order as that of said framing impulses, means for demodulating said`carri`er wave to produce picture signals, an amplifier preceding said demodulating means and comprising an amplifierv tube having a plurality of electrodes including a control grid and being of the variable-,mu type, means for deriving from said intercepted Vsignal at a point preceding said demodulating means a low frequency signal corresponding to said undesired modulation, and means for suppl-ying said low frequency signal to said controll grid with such polarity and amplitude as to substantially reduce said undesired modulation.

3. A television receiver for the reception of a carrier wave modulated by picture signals, by high frequency synchronizing impulses and by low frequency framing impulses, and cross-modulatedfby an undesired signal of low frequency as compared with the frequency of occurrence of said synchronizing signals but having a frequency which is of the same order as that of said framing impulses, said receiver being of the superheterodyne type including means for converting an incoming signal to an I. F. signal, means for detecting said I. F. signal and reproducing the picture represented by said picture signals, a control circuit connected to be supplied with said I. F. signal, rectifier means in said control circuit for deriving fromsaid I. F. signal a control signal corresponding to said undesired modulation, the time constant of said rectifier means being such that it does not derive picture signals from said I. F. signal, and means for supplying said control signal to at least one of said I. F. stages with such polarity and amplitude as to tend to cancel out said undesired modulation, said one I. F. stage comprising an amplifier tube having a non-linear grid voltage-plate current characteristic.

desired signal having a comparatively low frequency, said impulses being of greater amplitude than said picture signals and modulating said carrier wave in such direction as to increase its amplitude, said receiver comprising means for intercepting said modulated carrier Wave and for converting it to an I. F. signal, an I. F. amplifier for amplifying said I. F. signal, said I. F. ampliiier comprising an amplifier tube having a plu' rality of electrodes including a grid electrode and having a non-linear grid-voltage plate-current characteristic, an ampliiiertube having at least three electrodes including a control grid and a cathode, a con-denser, a resistor connected in shunt to said condenser, means for impressing said I. F. signal across said control .grid and said cathode through said condenser, the time constant of the circuit including said condenser and said resistor being such that said undesired signal only appears in the output of said last amplier, and means for supplying said undesired signal to the grid electrode of said I. F. amplifier tube with such polarity as to buck out said undesired signal modulation.

5. A television receiver for the reception of a carrier Wave modulated by picture signals, synchronizing impulses occurring at a comparatively high frequency, and nally by an undesired signal having a comparatively low frequency, said impulses being of greater amplitude than said picture signals an-d modulating said carrier Wave in such direction as to increase its amplitude, said receiver comprising means for intercepting said modulated carrier Wave and for converting it to an I. F. signal, an I. F. amplifier for amplifying said I. F. signal, said I. F. amplifier comprising an amplifier tube having a plurality of electrodes including a grid electrode and having a non-linear grid-voltage plate-current characteristic, an ampliiier tube having at least three electrodes including a control grid and a cathode, a grid resistor connected between said control grid and said cathode, a grid condenser, means for impressing said I. F. signal across said grid resistor through said grid condenser, the time constant of the circuit including said grid condenser and said grid resistor being such that said undesired signal only appears in the output of said last amplier, and means for supplying said undesired signal to the grid electrode of said I. F. amplifier tube with such polarity as to buck out said undesired signal modulation.

, "6. A television' system for transmitting pictures vfrom an aircraft ,driveny by propellers of electrically conducting material, said system comprising la television transmitter on said aircraft which includes means for radiating a carrier Wave which is modulated by video signals which signals include picture signals and periodically recurring synchronizing impulses of greater amplitude than picture signals of like polarity, said radiated Wave having the undesired characteristic that it is modulated by said propellers when they drive said aircraft, said undesired modulation occurring at a frequency within the frequency range of said Avideo signals, a television receiver for receiving said radiated carrier wave having said undesired modulation, means for demodulating said received wave to produce a video signal, an amplifier preceding said demodulating means,

said receiver further comprising means for deriving from said synchronizing impulses a control voltage which varies in accorda-nce with said undesired propeller modulation, and means for varying ythe gain of said amplifier in accordance with said f control voltage in such sense and amount as to substantially buck out said propeller modulation.

7. A television receiver for the reception of a carrier wave modulated by picture signals and by periodically recurring synchronizing impulses which impulses are of greater amplitude than picture signals of like polarity, means for intercepting said modulated carrier Wave after an undesired low frequency modulation has been superimposed thereon, means for demodulating said carrier Wave to produce picture signals, an amplifier preceding said demodulating means, means for deriving from said intercepted signal at a point preceding said demodulating means a low frequency control voltage corresponding to said undesired modulation, means for varying the gain of said amplifier in accor-dance With said control Voltage in such sense and amount; as vto substantially buck out said undesired modulation, and additional means for controlling the amplication of said intercepted signal before it is impressedupon said demodulating means,

said` additional means comprising an automatic volume control circuit which decreases said ampliflcation in response to an increase in the amplitude of the intercepted carrier Wave.

ALDA V. BEDFORD. 

